1. Field of the Invention
The present invention relates to a variable displacement reciprocating pump. The invention is described as a multi-plunger well service pump, but is not so limited since the invention can be used for a variety of applications and in a variety of arrangements, including single plunger pumps.
2. Description of the Related Art
Reciprocating pumps are widely used in a variety of applications. One application involves multi-plunger pumps for oil well service work. These pumps typically are high pressure pumps operating at pressures that range from low pressures to pressures as high as 15,000 psi. The pumping rate varies from low rates to more than 18 barrels per minute.
The pump prime mover, engine or electric motor, that powers the pump is normally coupled to the pump through a transmission. For purposes of this application, transmission will mean any device used between the prime mover and the pump to control the pump speed. Thus, the transmission could be manual or automatic shifted and could be multi gear ratio or variable speed, i.e. continuous. Thus a fixed ratio gear box that cannot be used to control the pump speed is not considered a transmission for purposes of this application.
The transmission allows the pump to pump at high rates and relatively low pressures when in “high” gears or at low rates and high pressures when in “low” gears. The horsepower is limited by the prime mover and the pump design. The typical transmissions have 5 or 6 possible gear ratios. The transmission used for a 500 hp multi-plunger pump cost about $30,000. In addition, the pump's lowest flow rate output is limited to the transmission gear ratio. Large volume pumps cannot reach the required low pump rates due to transmission ratio limits. Smaller pump and transmission arrangements that can reach the required low rates cannot meet the higher rates also required during well service work. Thus, two smaller pumps with accompanying engines and transmissions are typically required to meet the full range of rates and pressures needed in this type of work.
Thus, current multi-plunger well service pumps have two disadvantages. The first disadvantage concerns cost. Providing the pumps with transmissions and providing multiple pumps, engines and transmissions to achieve the required range of operating conditions is expensive, weighs more and takes up more space.
The second disadvantage of current multi-plunger well service pumps concerns performance. Pumps using current technology yield a discontinuous, stair step pressure-volume curve, have a limited working range, and are unable to be controlled by a computer.
The present invention addresses these problems by providing a single triplet pump that does not employ a transmission, but rather employs a means for varying the displacement of the pump to thereby provide the full range of operating conditions required for well service work. Thus, this system is less expensive since it eliminates the need for a transmission and eliminates the need for multiple pumps, engines, and transmissions. It furthermore can be computer controlled for improved performance while protecting driven components from excess input or over pressure.
The variable displacement pump's basic operation is similar to other reciprocating plunger pumps in that it employs a crankshaft with a connecting rod. The connecting rod is connected to a crosshead to which the pump plunger is attached. The big difference in the present invention over other reciprocating well service plunger pumps is that the amount of offset of the crank in the present invention is variable and that present pump does not employ a transmission as a means of varying the pumping rate of the pump.
U.S. Pat. No. 2,592,237 to E. H. Bradley recognized the desirability of using eccentric cams to obtain a stroke change for a plunger pump while the pump is operating. However, the means Bradley employed to change the relative positions of the cams was a rotating wheel that had to be grabbed by the operator while it was rotating and turned to change the stroke. In order for this to be done, the wheel had to be rotated at low speed, i.e. less than 60 rpm, so that the operator would be able to grab the turning wheel and rotate it in one direction or the other. If the operator's action on the wheel served to slow down the rotation of the wheel, this would either increase or decrease the stroke. To have the opposite effect of the stroke, i.e. decrease or increase the stroke, the operator would have to turn the rotating wheel faster than the wheel was already rotating. This method of adjusting the stroke of the plunger employed by Bradley is crude, is inaccurate, is limited in the speed at which it can be accomplished, and is potentially dangerous to the operator. Also, it is a method that could not be automatically controlled by a computer. Further, the Bradley pump does not have means to adjust a pump with more than one plunger.
Other positive displacement pumps, such as the one taught in U.S. Pat. No. 4,830,589 to Ramon Pareja, teach variable stroke positive displacement pumps, but these require the pump to be stopped in order to change the stroke. The design allows for adjusting the stroke for more than one plunger but the design was not suitable to the high horsepower required for oil field service pumps.
Also, Dowell/Schlumberger originally designed PG oil well service multi-plunger pumps, which are typical of the type of pumps currently used in the oil field. The Serva model TPA-400 is typical of this type of pump. These pumps use cams for driving the connecting rods. However, the cams of these types of pumps are not variable and therefore can not be employed to vary the stroke of their associated plungers. The output is changed by varying the speed of the input drive shaft powering the pump.
Although diesel engines are employed to power most land based multi-plunger pumps, it is common to drive multi-plunger pumps with electric motors in offshore operations since most of the rigs are operated with electric motors rather than diesel engines. Variable motors and either DC or AC controls are required for operating conventional multi-plunger pumps at different speeds. These variable motors and controls are very expensive. The present invention would eliminate the need for these expensive variable speed electric motors and controls since it would require only fixed speed electric motors to power it. This would reduce the cost and the complexity for electrically powered installations over what is currently required.
A variable displacement reciprocating pump, such as the present invention, increases the range that a given pump can operate by being able to adjust the stroke of the pump as needed without varying the operation of the prime mover that powers the pump. Having a variable displacement pump eliminates the need for a multi-gear transmission. The pump input shaft of the present invention can be held at constant or near constant speed. Although variable displacement pumps have been employed in hydraulic transmissions for approximately 50 years, the mechanism used in hydraulic transmissions is not suitable for oil field service pump.
The present invention employs a method of adjusting the relative relationship between the outer and inner eccentric cams to vary the offset of the crank and thereby vary the stroke of the pump. The mechanism that adjusts the cams of the present invention is considered novel. The present invention has an intermediate drive shaft with gears that is parallel to the variable cam or central shaft. The parallel intermediate shaft is used to simultaneously power all of the outer eccentric cams. This system of driving the variable cam is novel. The inner eccentric cams normally rotate together with the outer eccentric cams with no relative motion. The power to the cams is split. The stroke of the pump is adjusted by rotating the inner cams relative to the outer cams. The relationship of the outer cam relative to the input drive shaft is fixed whereas the angular position of the inner cam is variable. The relative position of the inner cams relative to the outer cams is changed with a rotating hydraulic rotary actuator that is located between the input drive shaft and the inner cams. The inner and outer cams turn together with no relative rotation when the pump stroke is not being changed. The hydraulic rotary actuator is also turning while the pump is being operated. The hydraulic rotary actuator is connected to a control mechanism through a swivel union.
In addition, the relative position of the rotary actuator, and thus the stroke of the pump, is measured by an electronic position sensor provided on the present invention. A position signal is transmitted to a readout device or computer via a rotary slip ring. An input shaft speed sensor transmits the input speed to the computer. The computer can then calculate the pump output flow from pump speed and stroke. Alternately, a flow meter can be employed to measure the flow directly. A pressure transducer on the discharge of the pump measures pressure. The computer can calculate hydraulic horsepower from the measured pressure and flow. Thus, the computer can be set to control the pump output with several optional conditions. The computer can limit any one or combination of pump output pressure, output flow, and horsepower. Conventional pumps drive pumps through transmissions with discrete gear ratios and thus cannot be controlled proportionally with respect to flow output. The present invention is continuously variable and therefore can easily be controlled through a proportional controller. The controller controls the position of the rotary actuator and thus the pump stroke.
Use of a variable displacement pump makes a number of control options possible. The pump is continuously variable from 0 to 100% displacement. Thus by employing a feedback position sensor for displacement in combination with a speed sensor, pressure sensor, and a computer, the control system can limit any one or combination of pump output pressure, output flow and horsepower.
At this point it should be noted that there is a relationship between flow and pressure. During almost all pumping operations, the pressure on the pump will be related to the pumping rate plus a factor for the difference in the fluid density inside the casing verses outside the casing. Thus, if the pumping rate is reduced, the pressure will automatically be reduced, also.
The control system can have a pressure override feature similar to hydraulic systems that causes the pump to pump at lower rates if a preset pressure limit is reached. A pressure override would be automatic and cause the pump to destroke until the pressure limit was satisfied, even if it required the pump to destroke completely. Thus, the present invention would limit discharge pressure by destroking rather than through interaction with the typical engine and transmission of prior art pumps. Computer controlled rates would be easily accomplished without the step-wise changes that occur when employing transmissions with fixed gear ratios. Also, the continuous variability of the present pump allows it to operate at lower flow rates than conventional pump and transmission systems.
Also, a pumping horsepower limit can be set in the computer. The control system would calculate the actual pumping horsepower and when the limit is reached, the pump could be destroked to reduced the flow and therefore limit the horsepower. This will be useful to keep the engine or a pump from being overloaded. It also will be useful when the same engine is being used to drive other systems. If, for example, the engine has a potential of 650 hp, the power consumed by the present multi-plunger pump can be limited to 500 hp thus always leaving a minimum of 150 hp for other systems, i.e. for operating hydraulics to drive centrifugal pumps. In prior art systems, it was common to use a separate engine to operate other auxiliary systems such as centrifugal pumps. This was desirable since the auxiliary engine could be maintained at a constant speed, thus insuring predictable performance for the centrifugal pumps. The engine used to drive the prior art multi-plunger pump is typically operated at different speeds due to the need to adjust pumping speed. When pump speed was changed, typically engine speed and gear ratios were changed. If the same engine was used to drive both the triplex pump and an auxiliary pump, for example a centrifugal pump, the performance of the centrifugal pump would be adversely affected when transmission gear changes were made due to the accompanying engine speed changes. With the present invention, a single engine with more horsepower can be used simultaneously for both the multi-plunger and centrifugal pumps without sacrificing performance of the centrifugal pumps. At the same time the present multi-plunger pump is protected from being overloaded.
Thus the present variable displacement pump system has the advantage being lower in cost and performing better than prior art pumps. It does this by eliminating the need for multi-speed transmissions and thereby reducing the overall cost of the engine, transmission, and pump package. The cost of the present pump should be considerably less than that of a conventional pump and transmission which currently sells for about $95,000.00.
Also, the present invention reduces the need to have two pumps by being able to operate the multi-plunger at low displacement values, i.e. low flow rates, while being able to meet the highest pump rate needed.
Further the present invention limits the input to the pump gearbox to engine torque. This is contrasted with prior art engine and transmission pump systems which increased the engine torque by transmission gear reductions. Thus the input maximum torque on the present pump will be up to eight (8) times less than prior art pumps. Conventional systems require the changing of transmission ratio to reduce pump speed, to reduce discharge flow and to increase maximum possible pressure. The present pump achieves both by changing the pump stroke. Reducing the pump stroke on the present invention reduces the pump flow output and reduces the torque required to obtain a given discharge pressure.
In addition, using the present invention, two pumps can be driven with the same engine without a transmission while one or the other or both of the pumps can be stroked per the needs of the job. The pumps would be independently controlled so the pumps could be operated at different flow rates and different pressures, and could discharge to different parts of the well, for example, to the inside of the casing and to the annular part of the casing. The computer control could be set to limit the horsepower of each pump so that neither pump could be overpowered.
This arrangement could also be used to build a double pump cementer with only one engine. Typically, a double pump cementer has three engines where the third engine is used to drive auxiliary systems. The auxiliary systems can be any hydraulic, mechanical or electrical system that has a need for power. With the opportunity to operate the engine at a constant speed, then a single engine could be used to drive two variable displacement pumps and also the auxiliary systems. This arrangement would be more compact, have a lower weight, be simpler to control, and be more economical than currently available systems. Also, one engine having a horsepower equal to three separate engines is also more economical to purchase than the three separate engines in addition to the cost savings resulting from not needing a transmission associated with each engine plus extra controls and instruments for multiple engines, transmissions and pumps verses a single engine pump system.
And, the present invention is able to adjust the pump stroke for a multiple plunger pump simultaneously while the pump is turning and pumping. The present pump allows relatively high power transmission, i.e. greater than 500 hp, as is required for well service operations.